As an ignition coil for an internal combustion engine, there is an ignition coil which includes: a primary coil; a secondary coil; a center core disposed on an inner circumference side of the primary coil and the secondary coil; an outer circumferential core disposed on an outer circumference side of the primary coil and the secondary coil; and an igniter that allows and blocks electrical conduction through the primary coil (for example, refer to Patent Literature 1). The primary coil, the secondary coil, the center core, the outer circumferential core, and the igniter are housed within a case. A connector unit which is connected to external devices is formed projecting from the case. The connector unit includes a signal terminal member for transmitting a switching signal to the igniter and a grounding terminal member for grounding the igniter, for example.
In the above-mentioned ignition coil, the above-mentioned outer circumferential core is connected to the grounding terminal member of the connector unit via a conductive member. Thus, the outer circumferential core can be grounded (earthed). When the outer circumferential core is grounded, for example, noise generated from the outer circumferential core can be prevented from occurring.
Particularly, in recent environment friendly engines, the output voltage of the ignition coil has been increased. Accordingly, the charged voltage of the center core and the outer circumferential core has been increased. Thus, in order to reduce the influence of the charged voltage of the center core and the outer circumferential core on the igniter, it is further desired that an ignition coil with a built-in igniter be configured to allow the outer circumferential core to have a ground potential. In addition, it is desirable that the center core and the outer circumferential core be allowed to have a ground potential easily at low cost.